Optical waveguide for frequency conversion

Abstract

A photonic chip for optical frequency conversion includes a substrate, and a planar optical waveguide disposed along a surface of the substrate. The optical waveguide has an optical core of periodically-poled thin-film ferroelectric material, the thickness of the optical core varying along the optical waveguide. The width of the optical core varies therealong in a manner correlated with variations of the thickness of the optical core along the optical waveguide, e.g. in a manner complementary to measured variations of the thickness.

We claim: 
     
         1 . An apparatus comprising:
 a photonic chip for optical frequency conversion, comprising:
 a substrate having a surface; and 
 a planar optical waveguide disposed along the surface and comprising an optical core of periodically-poled thin-film ferroelectric material, the optical core having a thickness and a width, the thickness varying along the optical waveguide; 
 wherein the width of the optical core varies along the optical waveguide in a manner correlated with variations of the thickness of the optical core along the optical waveguide. 
   
     
     
         2 . The photonic chip of  claim 1  wherein the width of the optical core varies along the optical waveguide in a complementary fashion to the thickness of the optical core. 
     
     
         3 . The photonic chip of  claim 1  wherein the ferroelectric material is lithium niobate. 
     
     
         4 . The photonic chip of  claim 1  wherein the ferroelectric material is lithium tantalate. 
     
     
         5 . The photonic chip of  claim 1  wherein the width varies such as to maintain a quasi-phase matching along the optical waveguide for pump light propagating in the optical core and light generated by non-linear conversion of the pump light in the optical core. 
     
     
         6 . The photonic chip of  claim 1  wherein the width varies by at least 20 nanometers over the length of the optical waveguide. 
     
     
         7 . The photonic chip of  claim 1  wherein the optical waveguide core comprises a sequence of core segments of different widths. 
     
     
         8 . The photonic chip of  claim 7  wherein at least some of the core segments have an approximately constant width along the length thereof. 
     
     
         9 . The photonic chip of  claim 7  wherein adjacent ones of the core segments are connected by wedge-shaped segments. 
     
     
         10 . The photonic chip of any one of  claim 7  wherein at least two of the core segments differ in width by at least 20 nm. 
     
     
         11 . The photonic chip of any one of  claim 7  comprising at least 10 adjacent pairs of the core segments, the core segments in each of the pairs being of different widths. 
     
     
         12 . The photonic chip of  claim 11  wherein at least two of the core segments differ in width by at least 20 nm. 
     
     
         13 . A method comprising:
 selectively etching a layer of ferroelectric material disposed along a planar surface of a substrate to form an optical core of a waveguide, the optical core having a width that varies along the waveguide in a manner correlated with variations of a thickness of the layer along the optical core of the waveguide.   
     
     
         14 . The method of  claim 13  comprising measuring the thickness of the layer at multiple locations along the waveguide. 
     
     
         15 . The method of  claim 13  comprising varying the width of the optical core along the optical waveguide in a manner complementary to the variations of the thickness of the layer. 
     
     
         16 . The method of  claim 13  comprising computing the width of the waveguide for a plurality of locations along the optical waveguide based on the thickness of the layer obtained for said locations. 
     
     
         17 . The method of  claim 14  further comprising periodically poling the ferroelectric material of the layer. 
     
     
         18 . The method of  claim 17  wherein the measuring is performed prior to the periodically poling. 
     
     
         19 . The method of  claim 17  wherein the measuring is performed after the periodically poling. 
     
     
         20 . The method of  claim 13  wherein the ferroelectric material comprises one of lithium niobate and lithium tantalate.